Optical waveguides can exhibit the property of photosensitivity manifest as a permanent, light-induced refractive index change. Initial experiments as described in U.S. Pat. No. 4,474,427 issued Oct. 2nd, 1984, invented by K. O. Hill et al demonstrated the phenomenon in optical fibers but recently it has been detected as well in planar glass structures, including, for example, silica-on-silicon and silica-on-silica planar waveguides.
Photosensitivity can be used to make retroreflecting Bragg gratings, mode convertor gratings and rocking rotators in optical waveguides; to fabricate such devices, a permanent, spatially periodic refractive index modulation is impressed with light along the length of the photosensitive core of the optical waveguide.
Methods of fabricating such Bragg gratings are described in U.S. patent application Ser. No. 969,774, filed Oct. 29th, 1992, invented by K. O. Hill, B. Malo, F. Bilodeau and D. Johnson and entitled METHOD OF FABRICATING BRAGG GRATINGS USING A SILICA GLASS PHASE GRATING MASK.
The near UV absorption spectrum of the Ge-doped core of optical fiber, Ge:SiO.sub.2 -on-Si or Ge:SiO.sub.2 -on-SiO.sub.2 waveguide is influenced strongly by the type and concentration of in-core defects. It has been found that photosensitivity in Ge-doped core waveguides is linked with absorption due to oxygen vacancy defects in the 240 nm UV region, as described by G. Meltz et al in Optical Letters 14, 823 (1989). High quality optical fiber such as Corning SMF-28 fiber and NTT silica-on-silicon planar guides contain comparatively low concentrations of defects. As a result both types of waveguide are relatively transparent in the near UV and are characterized as being weakly photosensitive.